Composite coatings have been made by a number of methods which are referred to generally as thermal spray processes. Thermal spray processes are used in numerous industries to form coatings on metallic and non-metallic substrates. The relative sophistication of these processes and of the coatings so formed has increased rapidly in recent years resulting in the fabrication of high-tech composite materials. In essence, discrete particles are heated (often melted or softened) and accelerated in a high energy stream. In this state, the particles impact a target. Under proper conditions, high quality coatings are formed. It will be appreciated by those skilled in the art that while a number of parameters dictate the composition and microstructure of the final coating, the nature of the particles which are sprayed determines in large part the characteristics of the coating. There has been, therefore, a continuing interest in developing new thermal spray powders and methods for making such powders.
Thermal spray powders are used in both plasma spraying and combustion flame spray processes. Plasma spraying employs a high velocity gas plasma to spray a material. The plasma is formed by flowing a plasma forming gas through an electric arc which partially ionizes the gas into a plasma stream. The recombination of ions and electrons then creates an extremely hot, high velocity gas jet exiting the plasma gun nozzle. Particles are injected into the gas either inside or outside the gun. The particles which are sprayed typically range in particle size from about 5 to 150 microns. The temperature of the jet may reach 10,000.degree. C. and the sprayed particles may attain supersonic velocity. In combustion flame spraying, a fuel gas and an oxidant gas are flowed through a nozzle and then ignited to produce a diffusion flame. The material to be sprayed is flowed into the flame where it is heated and propelled toward a substrate. The powder may be injected axially or externally into the flame in a carrier gas. Some flame spray guns utilize a gravity feed mechanism to introduce the powder into the flame front.
A number of prior art thermal spray powders and methods of forming thermal spray powders are known in the art. As stated, the characteristics of the powder are critical in determining the properties of the final coating. Moreover, powder properties also dictate whether a selected powder can be successfully sprayed in a particular thermal spray application. Although it is known to form composite materials by simultaneously spraying two or more materials, at times using two distinct thermal spray guns or multiple injectors, the use of composite powders is preferred. Thus, in a number of applications, composite coatings are formed by thermal spraying a powder which consists of individual composite particles.
Composite thermal spray powders suitable for thermal spray techniques may be either binderless or binder-containing powders. And such powders may consist of homogeneous powder particles wherein the two discrete materials are uniformly interdispersed (e.g., particles of one component uniformly and homogeneously dispersed in a matrix of the other component) or of clad particles (e.g., one component forming the core and the second component forming a surface coating on the core particle). Generally, homogeneous particles have been prepared in both the binder-containing and binderless forms. The clad particles formed with binders are also readily available. Binderless clad particles are generally not readily available. Currently only a few such binderless clad powders, which are prepared using chemical deposition techniques, are available.
Spray drying techniques have been used to prepare homogeneous, binder-containing particles. In this method, a slurry of two discrete materials suspended in a binder solution is sprayed into a heated chamber. The resultant dried agglomerated particles which contain binder are then classified by size. If the particle size of the two materials are about the same, the resultant powder is generally a homogeneous powder wherein the two discrete materials are completely and intimately interdispersed throughout the powdered product. Or if one of the particles is much smaller than the other, clad particles can be formed where the smaller particle forms a coating on at least some of the larger particles. In either case, the agglomerated powder is then sprayed utilizing one of the aforementioned thermal spray methods to form a composite coating.
U.S. Pat. No. 3,655,425 provides a method for producing a clad powder using a binder. In this patent, the cladding is accomplished by mixing the metal core particles and the ceramic cladding particles with a resinous binder in a suitable solvent. The solvent is then removed and any agglomerates formed between the core particles are broken up. The resulting particles consist of a metal core with ceramic particles attached to the surface through the binder material. The clad powder can be used to form a composite coating using thermal spray methods.
Binder materials may degrade coating performance. For example, it is known that thermal-induced changes may occur during thermal spraying at the interface of two different materials of a composite particle. As the materials chemically react or form an alloy layer, the capacity of the sprayed powder to form high performance coatings having excellent adhesive properties may be enhanced. The ability of the materials to interact in this manner, however, is inhibited by the presence of a layer of binder which physically separates the discrete materials. In other words, a binder may form a barrier to material interaction thus interfering with the fabrication of coatings having desired characteristics. Although organic binders may be employed which are vaporized or oxidized during the thermal spray process, vaporization or oxidation may not be rapid enough or complete. This is particularly true where plasma spraying is conducted under vacuum conditions or in an inert atmosphere, since conventional composite powders are formed with organic binders which generally do not fully vaporize or oxidize under these conditions.
Clad powders without added binders have been prepared using chemical deposition techniques whereby the coating is deposited from the appropriate deposition solution directly upon the seed or core particles. The preparation of such clad powders is described in V. N. Mackiw, W. C. Lin, and W. Kunda, "Reduction of Nickel by Hydrogen from Ammoniacal Nickel Sulphate Solutions," J. Metals 786 (1957). The selection of the components of such clad powders is very limited due to the limited availability of the required chemical deposition solutions and the requirement that the deposition process itself be carefully controlled. Such processes are generally very slow, thereby significantly increasing the cost of the clad powders.
Processes are also known for producing binderless powders of homogeneous particles where the components are uniformly dispersed throughout the powder (i.e., binderless non-clad particles). These processes include high energy ball mills, such as attritors, whereby the components are milled together for extended periods of time to form homogeneous powders. Generally, the metal powder and the powder component to be dispersed in the metal matrix are introduced into an attritor grinding mill which is a high energy driven ball mill with the powders and balls held in a stationary tank and agitated by rotating impellers. During milling the ingredients of the powder mixtures are reduced in size and brought into intimate contact by flattening and crushing the particles, welding them together, and repeating the process again and again. In effect, the powders are repeatedly torn or ripped apart (i.e., reduced in size) and recombined or built back up (i.e., fused or welded together) over an extended periods of time (normally 4 to 24 hours or even longer). Such techniques are often referred to as mechanical alloying. The resultant powders essentially consist of homogeneous and uniform distribution of the initial component within the powder particles U.S. Pat. Nos. 3,740,210, 3,816,080, 4,010,024, 4,101,713 4,300,947 4,705,560 4,722,751 and 4,749,545 provide representative examples of the use of high energy ball mills for producing homogeneous powders by mechanical alloying processes. High energy ball mills can also be used simply to reduce the particle size of a powder. Ultra fine particles having an average size of less than 5 microns may be produced using an attritor or a hammer mill over an extended time period.
U.S. Pat. No. 4,818,567 describes a process by which certain metallic coated particles are reported to be produced. The coated particles have relatively hard metal core material and a ductile and/or malleable metal coating material. In this method, the aspect ratio of a ductile and/or malleable metal is first reportedly increased to a high value (generally greater than about 50 to 1). The aspect ratio is defined as the ratio of the diameter of the particle to its thickness. The increased aspect ratio or essentially "flake" geometry can be achieved with relatively high speed vibratory, rotary, or attritor milling techniques. The resulting metal flakes are then reportedly applied to the relatively hard core material using a "mechanical smearing technique." The metal flakes and core material are reportedly milled in a low speed vibratory, rotary, or attritor mill "over an extended period of time until the ductile material has effectively coated the core metal particles through mechanical action." Coating materials include copper and copper alloys, aluminum and aluminum alloys, iron and iron alloys, nickel and nickel alloys, and lead and lead alloys. Core materials include iron and iron alloys, steel, stainless steel, and cobalt alloys. As noted, the core material must be sufficiently less deformable than the coating material so that the core material will hold its particle shape while the coating is applied.
Patent application Ser. No. 07/615,771 (Nov. 19, 1990) commonly assigned to the assignee of the present invention describes a method for preparing binderless thermal spray powders by mechanical agglomeration using a rotating drum with a treating member having an impact surface adjacent to the inner surface of the rotating drum. At least two powdered materials are placed in the rotating drum and are centrifugally forced against the continuously curved portion of the rotating drum, whereupon the powdered materials move between the impact surfaces of the treating member and the continuously curved portion of the drum. The forces of shear and compression acting on the powdered materials affect the mechanical agglomeration. The thermal spray powders produced by this method have the components dispersed uniformly throughout the particles. Experiments directed at preparing clad powders of the type of the present invention using the method of patent application Ser. No. 07//615,771 have not been successful for commercial applications.
Patent application Ser. No. 07/736,544 (Jul. 26, 1991) and commonly assigned to the assignee of the present invention describes a method for producing composite powders containing hexagonal boron nitride and aluminum or aluminum/silicon alloys where the components are comparable sized, finely-divided, and uniformly distributed. The composite powders may contain a binder or may be binderless. The binderless composites are generally prepared using the method described in patent application Ser. No. 07/615,771 discussed above.
Patent application Ser. No. 07/792,533 (Nov. 13, 1991) and commonly assigned to the assignee of the present invention describes a method for producing composite powders containing hexagonal boron nitride and metal alloys where the components are comparable sized, finely-divided, and uniformly distributed. The composite powders may contain a binder or may be binderless. The binderless composites are generally prepared using the method described in patent application Ser. No. 07/615,771 discussed above.
Although much effort has been directed towards preparing thermal spray powders, there still remains a need for binder-free agglomerates or composites, especially binder-free clad powders, which can be used as thermal spray powders. There still remains a need for a method of forming thermal spray powders, especially clad powders, which are binder-free and which have superior mechanical and chemical characteristics. And there remains a need for a simple and direct method of forming binderless clad powders in which the composition of the clad powder can be easily varied to provide a wide variety of composite coatings using thermal spray techniques. The present invention addresses these needs and others.